The fuel assemblies of nuclear reactors, such as pressurized-water nuclear reactors, comprise a framework in which fuel rods of great length are disposed in order to form a bundle The framework comprises spacer grids which are spaced relative to one another along the length of the assembly and connected together by guide tubes.
Each of the spacer grids comprises an assembly of cells each intended to receive a fuel rod and disposed in a regular network, generally a squared mesh.
The network forming the grid is surrounded by a frame whose transverse section corresponds to the section of the assembly and which consists of small plates assembled together, for example in the form of a contour of squared form.
The small plates forming the frame of the spacer grid are cut along their longitudinal edges to form fins which project relative to the upper lower faces of the spacer grid.
The fins are folded towards the inside of the grid along the edge of the corresponding small plate and so as to form a perfectly defined angle with the latter.
The fins of the spacer grids are intended, on the one hand, to facilitate the guiding of the fuel assembly when it is positioned in the core of the reactor or in a storage cell and, on the other hand, to ensure mixing of the primary coolant circulating in contact with the fuel assembly during operation of the reactor.
The guide fins of the spacer grids of the fuel assemblies are inclined inwards so as to prevent the grid of the assembly from hooking onto the structure of the internal equipment of the tank of the reactor or onto an adjacent fuel assembly during refuelling or discharging of the core of the reactor.
The fins of the spacer grids of the fuel assemblies may be deformed or folded under the effect of impacts which occur during their handling or produced by foreign bodies entrained by the coolant and circulating at high speed during operation of the reactor.
After a period of operation of the order of one year, the fuel assemblies contained in the tank of the reactor and forming the core may be examined, before being refuelled into the core, in order to determine whether any of them have been damaged.
Generally, only one-third of the assemblies of the core are renewed, but all the fuel assemblies are discharged in order to permit checking, for example inside the tank of the reactor.
The fuel assemblies are placed under water in a pool, such as a storage pool, in order to be examined before their possible refuelling into the core.
After an extended operating time, the guide fins of the spacer grids of some assemblies may be deformed, for the reasons given above.
In order to be able to reuse the fuel assemblies whose spacer grids may have deformed fins, it has been proposed to compensate for the fins which are deformed or folded at an angle different from their defined angle of inclination by completely folding the fins which are deformed or in an incorrect position in order to press them against the wall of the corresponding grid. However, when this is done, the fins can no longer fulfil their role in respect of guiding the assembly nor in respect of mixing the primary cooling fluid circulating in contact with the assembly.
Moreover, this operation of completely folding the fins runs the risk of leading to their breakage along the folding line before reaching the final position of the fin in contact against the small plate. This risk is greater when the metal of the small plate has been rendered less ductile due to irradiation.
Moreover, the devices proposed for performing these operations of completely refolding the fins are ill-adapted to implementation under a depth of water which is greater than the minimum biological protection, which corresponds to a depth of 3 meters.